The following points highlight the top eight evidences of organic evolution of life. The evidences are: 1. Palaeontological Evidence 2. Geographical Evidence 3. Morphological Evidence 4. Embryological Evidence 5. Taxonomic Evidence 6. Physiological Evidence 7. Evidence from Artificial Selection 8. Evidence from Recent Progress of Evolution.
1. Palaeontological Evidence:
Palaeontology is a branch of Geology which is concerned with the study of fossils. Fossils are long-preserved remains of plants and animals which are found buried in the earth. They may be petrified skeletons or shells which have been converted into stones.
They may be foot-prints of animals or imprints of other parts such as leaves of a plant. They may be intact fossils such as the mastodons preserved in the frozen ice of Siberia. They may be insect, preserved in crystal clear amber.
They may be petrified cast of an individual from the interior of which the individual itself has disappeared by decay. Most of the fossils were formed when a section of earth-containing buried organisms were gradually converted into stone by geological changes.
In such a section the oldest layer is evidently the deepest, and the youngest is the topmost. But as the years rolled by, this arrangement may be disturbed by various geological upheavals, such as cracking and erosion of the upper layers, thereby exposing the buried fossils.
Embedded in the earth along with fossils there are radioactive substances, such as Uranium, which undergoes decay and is changed into Lead within a definite period of time. Thus by measuring the quantity of these substances in a given layer, the age of the crust can be determined accurately along, with the age of its fossil records.
Judged on this basis the first fossils are about 900 million years old, and these were unicellular plants and animals. Calculating in this manner the various crusts of the earth are divided into five eras, and each era is further subdivided into a number of periods.
The fossil treasure of these eras and periods clearly indicate that plants and animals became more complex with the passing of time. In the crust just above the remains of the unicellular organisms, fossils are abundantly represented by diverse forms. This layer is 450 million years old and contains fossils of all the invertebrate groups.
Fish-like vertebrates appear in the next higher layer which is 380 million years old. This was followed in successive layers by amphibians (350-300 million years), reptiles (300-150 million years), mammals (180-150 million years), birds (150-120 million years) and man (1 million years).
It is to be noted that members of each group at the beginning of its appearance were different from the plants and animals of today. They were mostly queer forms.
Some of them were ancestors of later types, others were intermediate forms, and a few were highly specialised which eventually became extinct without leaving any descendants. Prominent amongst the latter were huge dinosaurs, a group of ancient reptiles, some of which measured 24 metres from snout to tail and stood 12 metres high.
Some dinosaurs, such as the Ichthyosaurus were aquatic types; others, such as the Pterodactylus took to the air. Where are these monsters which ruled the earth only 150 million years ago? We know them today only by their fossils. Why did they become extinct? Probably the tyrants had to pay the price for becoming over specialised.
The intermediate forms of fossils may be cited as ‘missing links’ between two different groups because they possessed features common to both. The most famous amongst such transitional forms is the fossilized bird Archaeopteryx which was found in Switzerland about 100 years ago. This ancient fossil bird is 150-120 million years old.
It possessed wings and feathers which are characteristics of birds, yet it had teeth like the typical reptiles and possessed a lizard-like tail. Evidently Archaeopteryx is an unmistakable example of a transitional form linking the birds to the reptiles.
Similarly, a group of fossil plants, the Pteridospermae, serves to connect the pteridophyta with the spermatophyta. The fossilized Pteridospermae possessed characteristics of both the groups. They therefore link the ferns with the modern seed-bearing plants.
In addition to the history of origin of large groups of animals and plants, palaeontology records the history of origin of certain modern species such as camels, elephants and horses. A comprehensive document of the fossil history of the horse is represented diagrammatically in Fig. 188.
The ancestors of the modern horse, Eohippus, lived about 60 million years ago. It was only 27 cms. in height, and it had four toes on the fore feet and 3 toes on the hind feet. Every subsequent layer of the earth’s crust contain slightly different types of horse fossils. They reveal a steady increase in size, particularly the height of the animal, with a progressive reduction in the number of toes.
The Mesohippus which was found at a higher layer stood 54 cms high and had three toes in each foot, the side toes touching the ground. Merychippus and Pliohippus which were found in successively higher layers had three toes on each foot, side toes no longer touching the ground.
Finally came the fossil of the modern horse Equus with single-toed foot, the side toes being reduced to mere vestigial flints. Equus stood full 150 cms high. The fossil horses prove that the present-day Equus is the child of the pre-historic Eohippus. This means that ‘descent with changes’ has taken place. The documentation of the horse fossils is a direct evidence in favour of the evolution theory.
Since fossils were formed only under certain circumstances and since most of the earlier fossils were destroyed by geological upheavals, the palaeontological record book is more or less incomplete. It is an old book with some of its pages worn out and a few completely torn away, particularly from the first chapters.
But in spite of this the palaeontological record book is sufficiently complete to prove:
(i) That the plants and animals were changing through the past ages;
(ii) That the earliest were the simplest, and the complex organisms appeared progressively in successive ear;
(iii) That there had been transitional or intermediate forms connecting two large groups, such as ferns with flowering plants or reptiles with birds;
(iv) That there were intermediate forms connecting a modern species directly to its pre-historic ancestral form, for example, the intermediate types linking the present-day horse Equus to the pre-historic Eohippus which lived 60 million years ago.
2. Geographical Evidence:
Each corner of the earth has its own contingent of flora and fauna. Different regions are inhabited by different forms, and the climatic condition of a country plays very little part in the present-day distribution of plants and animals.
Elephants for instance are found in India and Africa but not in Brazil although Brazil is as much tropical as the other two countries. Furthermore, the African elephants are somewhat different from their Indian brethren.
New Zealand and Great Britain are almost alike in climatic condition but the living forms inhabiting the two places are widely different. Again representatives of a group may occur in widely separated places, but not in the regions in between them.
For example, Llama and Alpaca, which are related to Camels reside in South America while the Camels are inhabitants of Arabia and Central Asia. It has been proved that the ancestral form of the Camels is really American. It migrated westward to Asia across a land bridge which used to connect America with Asia in the pre-historic time.
From such anomalies in the distribution in space it may be inferred:
(i) That modern forms originated in the past from a common ancestral type;
(ii) That the ancient ancestral types, due to their migratory habits, went far away from their original home unless they were prevented to do so by the presence of some obstacle or barrier in their path of dispersal;
(iii) That the barriers may be physical or physiological;
(iv) That having reached a new country they were forced to live in a new environment;
(v) That they slowly underwent modifications in order to adapt themselves to the new conditions and those which were completely successful ultimately changed so much that they were transformed into new species;
(vi) That those which were partially successful changed into weak intermediate forms which ultimately perished and became extinct unless they were protected by the formation of a strong barrier.
It has been observed by travellers that animals residing on islands are similar to but specifically distinct from those of the nearest mainland, and that greater the distance between the two places the greater the difference in their faunas.
Thus the Galapagos group of islands, situated at a distance of about 805 kilometres away from the nearest American coast, has a fauna which is typically American but most of the species are restricted only to the islands, and there are some species which are more or less confined to a single island of the archipelago.
This fact also suggests ‘descent with modification’. Ancestral forms migrated to the islands from the nearest mainland, and having arrived there they were isolated and slowly transformed into new but related species.
3. Morphological Evidence:
(1) Comparative anatomy of the vertebrates reveals a close similarity in structural planning. The slight modification in detail is the effect of adaptation to different modes of living. For example, if we compare the heart of the fish, toad, lizard and guinea-pig we find that this organ is built on the same general plan in every case.
The two-chambered heart of the fish is very simple and admirably adapted to its aquatic environment. In the toad, though basically the same, the heart has modified a bit in response to the change of habitat.
Thus the two-chambered heart of the fish has been modified into the three-chambered heart in the amphibian, but even this is not sufficient to check admixture of the arterial and venous blood. In the lizard, which is a reptile, the heart is practically three-chambered; there are two auricles, but the single ventricle is partially divided by an incomplete partition.
In the guinea-pig, which is a mammal, the heart is four- chambered; there are two auricles and two ventricles, and this four-chambered heart is perfectly adapted to separate and direct the two kinds of blood to their respective destination (Fig. 189). The same is true about other organ systems.
In all vertebrates the nervous system includes the brain with five comparable subdivisions. The brain, however, exhibits progressive changes from fishes to mammals; there is gradual enlargement of the cerebral hemispheres—particularly the pallium which is the seat of intelligence, and also of the cerebellum which is concerned with muscular co-ordination (Fig. 190).
(2) Homologous structures, such as the forelimbs of the vertebrates, appear to differ widely from one another. In the horse they are required for locomotion on dry land. In birds they are modified to form the wings for flying.
In the whales, the forelimbs take the form of paddle-like flippers for swimming in water. In man the forelimbs are typically pentadactyle and adapted for grasping objects, and for fine manipulations.
But an examination of the skeletal architecture of these different types of forelimbs clearly indicates that they are all pentadactyle in nature and consist of practically the same bony elements and are all built on the same fundamental plan (Fig. 191).
Similarly the green stem of a plant, the tuber of a potato, the thorn of Duranta and the tendril of Passiflora may appear to differ widely from one another but all of them are homologous structures and the difference in appearance is due to difference in function.
(3) Vestigial organs are useless structures of reduced size, which are found in many animals. These structures are not functional in a given species but are homologous with functional structures in other closely related species. The muscles concerned with the movement of the pinna in man serve as a good example of vestigial organ.
These muscles are useless and small because man does not require to move his ear for catching sound-waves. In related lower forms, such as guinea-pig, these muscles are well-developed useful structures which turn the pinna towards the source of the sound.
The useless vermiform appendix in connection with the caecum of man is another example of a vestigial structure which is homologous to the well-formed and useful appendix of the rabbit (Fig. 192).
It seems rather difficult to account for the presence of such useless structures in man unless we assume that they represent vestiges of a useful functional organ in some remote ancestral form.
The greatly reduced vestigial wings of the running birds such as kiwi and ostrich and the vestigial hind limbs of the snake python are other examples. The scale leaves on rhizomes and tubers, the staminodes or stamens without anthers, are examples of vestigial organs in plants.
(4) Intermediate forms linking closely related groups ought to be present if each individual is to evolve out of the pre-existing lower form. Unfortunately such intermediate forms are invariably weak and often fail to survive. At least one animal, the duck-billed mole (Ornithorhynckus) is worthy of mention in this connection.
This is found in Australia and Tasmania, along with other ancient forms such as the spiny ant-eater (Echidna) and the Kangaroos. It has survived there because competition is very much reduced owing to the early isolation of Australia from the mainland before the origin of the higher mammals.
The duck-billed mole is undoubtedly a mammal, since it is hairy and suckles its young. But like the reptiles, it has a large T-shaped interclavicle in its pectoral girdle and it lays heavily yolked eggs. It may, therefore, be regarded as an intermediate form linking the reptiles to the mammals.
4. Embryological Evidence:
Study of the developmental history of any individual plant or animal is a convincing evidence in favour of the evolution theory. Every organism during its own development repeats the evolutionary history of its ancestors.
We are familiar with the tadpole larva of the toad which resembles the fish-like ancestors of the class amphibia to which toad belongs. Even in chick and human embryos there is a brief stage in which rudimentary and non-functional gill slits appear for a short time (Fig. 193).
The occurrence of a fish-like stage with gill slits in the toad, chick, and mammalian embryos can only be explained on the ground that they have descended from a fish-like ancestral stock.
The embryos cannot but recapitulate the centuries of slow evolutionary changes through which their ancestors had to pass. They, however, have cut down the process to a few days, quickly adapting themselves, during their embryonic development, to a new mode of life in a new environment.
Thus the adults of two species of animals may resemble each other very little, but their embryos may do so very strongly. For example, early stages of chick and rabbit are almost indistinguishable whereas the adults are easily distinguished from each other. Similarly the adults of annelid worms and molluscs differ widely, but the trochophore larva is common to both.
Sometimes we are able to detect relationship in the embryos, thereby establishing relationship between the adults. Sacculina, which is a parasitic crustacea found attached to the abdomen of the crab, is not recognisable as an arthropod animal, but its larva closely resembles other arthropod larvae. Similarly tunicates are recognised as chordates by their tadpole larva.
Embryological studies of the various plant groups support the doctrine of organic evolution. The fern spore, on germination, produces a green alga-like structure which quickly develops into a thalloid body like the liverworts; finally the pro-thallus is replaced by the mature fern plant.
In Acacia moniliformis, the seedling bears foliage leaves like other Acacias; but these are quickly shed off and the petioles expand to produce the leaf-like phyllodes. Such observations led Haeckel to postulate his Recapitulation theory which states that ontogeny, that is development history of an individual, recapitulates phylogeny, or developmental history of the whole race.
Although the recapitulation theory, as such, is not accepted by modern biologists, yet embryological studies have helped to detect vestigial structures such as the transient gill slits of land vertebrates, thereby linking them to the aquatic forms in which gill slits are useful structures.
5. Taxonomic Evidence:
The mere fact that we have been able to classify plants and animals into distinct groups is a significant evidence in favour of evolution. Modern classification is based on structural similarity, and this indicates close kinship. Hence the plants and animals belonging to any particular group are related to one another and evolved out of a common ancestral form.
The same statement holds good when we consider the inter-relationship between the phyla. It must be noted however that the evidence from classification is really an extension of the morphological evidence.
6. Physiological Evidence:
Closely related animals possess the same kind of enzymes and hormones whose physiological actions are identical. Trypsin, for instance, is secreted by the pancreas. It is a proteolytic enzyme which breaks proteins into polypeptides.
Trypsin secreted by a monkey is identical in chemical action to that of man, because man and monkey are closely related. The same is true for the hormone insulin which regulates carbohydrate metabolism and prevents diabetes in man.
Chemical studies of blood in closely allied species add further evidence in establishing relationship. It has been proved that haemoglobin crystals prepared from the blood of man and gorilla are identical, but these crystals differ from those derived from a horse or a dog.
Thus comparative physiology supports the theory of evolution, because closer the relationship between two animals the greater is the resemblance between the chemical processes which occur within them.
Occasionally comparative biochemistry helps to establish relationship between greatly divergent groups of animals. For example the starfish, an invertebrate, differs widely from the vertebrates, but the two phyla agree with one another in the chemistry of their muscular contraction. Similarly, the universal occurrence of cytochrome in the protoplasm of all aerobic organisms prove that they are related to one another.
7. Evidence from Artificial Selection:
Entirely new types of animals are often produced artificially under domestication. By careful selection of types which pleased him most or suited his purpose, man has produced numerous improved types of dogs, pigeons, horses, and cattle, which are structurally different from the wild ancestral stock.
The diverse types of domestic pigeons are all derived from the blue rock-pigeon, Columba livia, by artificial selection. If it is possible for man to produce changed types of animals within a brief span of time, it is equally possible for the nature to do so within several millions of years.
8. Evidence from Recent Progress of Evolution:
The best evidence for evolution will be to see the process in action. In a few instances, actual progress of evolution has been recorded by men who are even now living.
For example, the normal form of the peppered moth has light mottled wings. It was the prevailing type and was found everywhere in the countryside of England less than 100 years ago. Black mutant types would appear occasionally, and these being conspicuous were quickly destroyed by birds.
The black melanic types therefore could not get a strong foothold except in industrial towns such as Manchester where they were camouflaged by dust, soot and smoke. With rapid increase of industrial towns, the melanic type has completely replaced the light coloured type. This change-over has occurred recently within the memory of living men.
Evolution must not be regarded as an event of the past. The process is still in progress, and probably there is no final end. There is no one today who can seriously challenge the fact that evolution has occurred. The occurrence of evolution therefore is not a theory.
